Shading contributes to Sphagnum decline in response to warming

Area of Science:

• Ecology
• Plant Physiology
• Biogeochemistry

Background:

• Experimental warming of ombrotrophic bogs in Minnesota has led to decreased Sphagnum moss productivity and coverage.
• This decline impacts ecosystem carbon balance and biogeochemistry.
• Shrub expansion, linked to warming, increases shading, potentially affecting Sphagnum.

Purpose of the Study:

• To investigate the independent effects of reduced light and shrub litter on Sphagnum moss productivity.
• To differentiate the impacts of shading versus litter deposition on Sphagnum growth.

Main Methods:

• Controlled shading experiments using neutral density shade cloth (30%-90% light reduction) on hummocks and hollows.
• Measurement of Sphagnum growth, CO2 exchange, and chlorophyll-a fluorescence under varying light conditions.
• Assessment of shrub litter's impact on Sphagnum growth and soil chemistry.

Main Results:

• Sphagnum growth declined linearly with increased shading in hollows; no response was observed on hummocks.
• Shading induced etiolation in Sphagnum, resulting in thinner, spindlier growth under deeper shade.
• Shrub litter did not inhibit Sphagnum growth or increase base cation levels.
• Photosynthesis rates were significantly higher in Sphagnum exposed to 30% shade compared to 90% shade.

Conclusions:

• Increased shading from shrub expansion, a consequence of experimental warming, is a significant factor contributing to reduced Sphagnum growth.
• While direct warming and drying are primary drivers, the indirect effect of shading plays a crucial role in Sphagnum decline.
• These findings highlight the complex interactions between warming, vegetation changes, and Sphagnum moss health in bog ecosystems.